A wide variety of gaskets are known for use in sealing applications. Expanded polytetrafluoroethylene (PTFE) is widely used today as a gasket material. As disclosed in U.S. Pat. No. 3,953,566 to Gore, this material has numerous properties making it highly desirable as a gasket. These properties include being readily compressible and conformable, being chemically resistant, having relatively high strength, and being far less prone to creep and loss of sealing pressure than non-expanded, non-porous PTFE alone.
In many sealing applications, the gasket is used to seal the junction between flanges, such as between pipes. In such applications, expanded PTFE is a desirable material for the gaskets because the expanded PTFE gasket can be placed between the flanges, and the flanges can then be pressed together with the application of force, such as by tightening of bolts. This application of force compresses the expanded PTFE. As the expanded PTFE is compressed, its initial pore volume is reduced, thus densifying the expanded PTFE. Particularly with metal-to-metal flanges, it is possible to apply sufficient force (or “stress”) to the flanges to fully densify the expanded PTFE. Thus, in at least part of the expanded PTFE gasket, the pore volume is reduced to substantially zero, such that a fluid contained within the pipes is prevented from leaking between the flanges by the densified, non-porous PTFE gasket, which seals the flanges.
In many applications, particularly when harsh chemicals are used which would readily break down the metal or the metal could contaminate the chemical which is being transported or housed, it is common to use glass-lined steel, glass, or fiberglass reinforced plastic (“FRP”) piping and vessels. Because this equipment is so often used with extremely harsh chemicals, there is great desire to use PTFE gaskets to seal the connecting flanges of this equipment because of the well known extraordinary chemical resistance of PTFE. Unfortunately, non-expanded, non-porous PTFE gaskets are generally not conformable enough to effectively seal this type of equipment. In the case of glass-lined steel flanges, although there is a relatively smooth finish, there is often a large amount of unevenness or lack of flatness associated with the flanges. This unevenness or lack of flatness requires the gasket to have to conform to large variations around the perimeter as well as between the internal and external diameter of the flange in order to create an effective seal. Thus, a non-expanded, non-porous PTFE gasket is not conformable enough to seal many of these applications.
Because expanded PTFE is so conformable, it would be desirable to use expanded PTFE to seal these commonly uneven flanges. Unfortunately, in many of these applications it is not possible to apply sufficient force to the flanges to create enough gasket stress to fully densify the expanded PTFE gasket to create an effective seal. For example, glass-lined steel piping flanges, glass flanges, or FRP piping flanges may deform, fracture, or break upon the application of a high amount of stress. Thus, in these applications, an expanded PTFE gasket may not be completely densified to reach a non-porous state, and therefore does not become leak proof, because the maximum stress that can be applied to the flanges without breaking them is not sufficient to so densify the gasket.
U.S. Pat. No. 6,485,809, in the name of Minor et al., teaches a low stress to seal gasket construction comprising a multilayer, unitary gasket including at least one inner layer of expanded PTFE disposed between a first substantially air impermeable outer layer and a second substantially air impermeable outer layer, and a substantially air impermeable region bridging the first and second substantially air impermeable layers. By “low stress to seal” is meant a gasket which provides a substantially air tight, or air impermeable, seal upon the application of a relatively low stress (i.e., a stress below that required to fully densify a porous expanded PTFE gasket, generally less than about 20,700 kPa (3000 psi)). The Minor et al. gasket forms a substantially air impermeable seal when compressed at low stress. This patented construction overcomes many challenges in creating a desired low stress to seal gasket. However, improvements to such a construction are still desirable.
While expanded PTFE materials exhibit much better performance than non-expanded, non-porous PTFE in gasket applications, it still exhibits some propensity to creep, or flow, under load. Thus, expanded PTFE materials with improved performance characteristics are desirable.
It has been taught that PTFE structures can be stretched above the melt temperature to impart improved properties to the PTFE. U.S. Pat. No. 2,776,465, to Smith, for example, teaches stretching non-expanded PTFE materials at temperatures above about 325° C. to achieve higher tenacity and modulus in the resulting structures.
U.S. Pat. No. 5,814,405, to Branca et al., teaches heating an expanded amorphously locked article to a temperature above the crystalline melting temperature of the PTFE and stretching in at least the direction orthogonal to the direction of stretch carried out below the melt temperature. The resulting articles exhibit a microstructure which can be characterized as having highly elongated nodes with aspect ratios greater than 25 to 50, preferably greater than 150. Such membranes also exhibit high air flow and high strength. These membranes are useful as filters in filtration devices, as scaffolding for holding reactive or conductive fillers and as support layers in composite constructions.
A need has existed, however, for improved expanded PTFE materials with enhanced properties to meet the ever-increasing demands for improved gasketing performance, as well as for other high performance applications.
Particularly, while it is desirable to provide low stress to seal characteristics in a gasket; it is also important from a performance perspective for a gasket to exhibit good bolt load retention, which is a measure of the resistance to stress relaxation of the gasket material. The amount of leakage associated with a gasketed assembly is dependent on the amount of compressive load, also known as bolt load, on the gasket. “Bolt load retention,” as used herein, is intended to refer to the retention of the compressive load supplied to a gasket through a pair of flanges from the tightening of the bolts or clamps used to fasten the pair of flanges. Typically, the higher the bolt load on a gasket the lower the leakage will be from that gasketed assembly. PTFE gaskets (i.e., greater than about 50% PTFE, by weight) are prone to creep and stress relaxation when subjected to a compressive load. Reducing the amount of creep and stress relaxation in the gasket material results in higher bolt load retention by the gasket. Higher bolt load retention in the gasket produces a tighter seal with less leakage over the life of the gasket.
Thus, what has been desired for many years is an easy-to-use, highly chemically resistant gasket which can effectively conform to flange surfaces and sustain a high bolt load retention to maintain a tight seal over the life of the gasket. Accordingly, a purpose of the present invention is to provide not only such high performance gaskets, but also to provide improved expanded PTFE materials which can be used to achieve these goals.
Further, it has been desired to provide a highly chemical resistant, highly conformable gasket which not only has the ability to seal at low loads, but also sustains a high bolt load retention even when subjected to high loads. Such a universal gasket could be effectively used for both low load applications such as glass-lined steel, glass and FRP piping and vessels, as well as for high load applications such as with metal piping and vessels.
These and other purposes of the present invention will be provided herein.